1. Field of the Invention
The present invention relates to a lithium-air battery and a lithium anode composite of the lithium-air battery.
2. Related Art
In anticipation of the popularization of electric vehicles, an air battery having an energy density far higher than that of a lithium-ion battery is expected to be developed. The air battery uses oxygen in the air as an active material for cathode.
Incidentally, there is known a lithium-air battery using metallic lithium, an alloy composed primarily of lithium, or a compound composed primarily of lithium for an active material for anode. When attention is focused on the types of electrolytes, the lithium-air battery is roughly classified into an aqueous electrolyte type and a nonaqueous electrolyte type. A nonaqueous electrolyte type lithium-air battery is a mainstream of research and development since lithium-ion battery technologies, except an air electrode, can be utilized for the lithium-air battery.
On the other hand, research and development, though small in the number of instances, is also being conducted on an aqueous electrolyte type lithium-air battery. The aqueous electrolyte type lithium-air battery has such advantages that the battery is insensitive to airborne moisture and the electrolyte is inexpensive and nonflammable, compared with the nonaqueous electrolyte type lithium-air battery. Metallic lithium which is an anode active material reacts with oxygen and water, however, if the metallic lithium is brought into direct contact therewith. Accordingly, the aqueous electrolyte type lithium-air battery has to be provided with a protective layer of a polymer electrolyte, a lithium ion-conducting solid electrolyte, or the like, in order to protect metallic lithium against the atmosphere and an aqueous solution.
Hence, there has been proposed a lithium-air battery provided with an anode composite in which a buffer layer of a polymer electrolyte is formed on one surface of plate-shaped metallic lithium, and one surface of the polymer electrolyte buffer layer is covered with glass ceramics having lithium ion conductivity and serving as a water-resistant layer (see, for example, Patent Document 1 (Japanese Patent Laid-Open No. 2010-192313) and Non-Patent Document 1 (“Present Status and Issues for Lithium/Air Battery Using Aqueous Electrolyte,” by Yasuo Takeda, Nobuyuki Imanishi, Osamu Yamamoto; GS Yuasa Technical Report, Vol. 7, No. 1, pp. 1-6 (June, 2010)).
The conventional air battery described in Patent Document 1 or Non-Patent Document 1 is sealed in a container or a laminate film with one surface of a single air electrode directly facing one surface of a single anode composite. In such conventional air batteries, when it is required to increase an input/output density (output per weight), a number of air batteries having the same structure is simply used, or an air battery is simply upsized with the structure thereof unchanged.
However, the mode of simply using a number of air batteries having the same structure or simply upsizing an air battery with the structure unchanged, the mounting space of the air battery or batteries is inefficiently and significantly increased. Therefore, such mode is impractical where the air battery or batteries are mounted on an electric vehicle, for example.
Furthermore, in the air battery described in the above-mentioned Non-Patent Document 1, an anode composite is sealed or enclosed in a gas-barrier laminate film having a trilaminar structure composed of polypropylene (PP), aluminum foil and polyethylene terephthalate (PET). Moreover, in order to secure lithium ion conductivity inside and outside the laminate film, the anode composite of the Non-Patent Document 1 is configured such that openings created in the laminate film are plugged up with glass ceramics serving as a lithium ion-conducting window material.
It is, however, difficult to bond the polypropylene (PP) of the laminate film and the glass ceramics of the anode composite to each other with an adhesive agent, thus resulting in the lack of durability. In addition, in order to heat-weld the laminate film to form the anode composite, a welding margin of approximately 10 mm is required in an outer peripheral portion of the laminate film. This requirement results in an expansion in the area of the laminate film and an increase in the volume of the air battery, thus being inconvenient.
That is, the conventional air battery is for the purpose of providing an experimental small-sized unit cell, and it is therefore difficult to configure a compact practical cell having increased battery characteristics, in particular, an energy density.